One of the major sources of nitrogen oxides (NO.sub.x), carbon monoxide (CO), and hydrocarbon (HC) emissions is the combustion of fossil fuels in internal combustion engines. Catalytic controls have been used effectively for many years to control these emissions from automobiles and other rich burning internal combustion engines. However, it is now desirable to control similar emissions in lean burning engines, such as diesel and dual fuel engines which can run on diesel fuel or natural gas.
The formation of nitrogen oxides in internal combustion engines is primarily caused by the oxidation of nitrogen in the air within the combustion chamber of the engine. While both nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are formed as final products, nitric oxide is the predominant product emitted. The three factors that contribute most to nitrogen oxide formation are high combustion temperature, pressure, and oxygen availability.
The formation of carbon monoxide in internal combustion engines is a result of the incomplete combustion of fuel which occurs when there is insufficient oxygen near the hydrocarbon (fuel) molecule during combustion. Also incomplete combustion could be caused by the quenching of the hydrocarbons near a cold surface in the combustion chamber. Since the diesel and the natural gas fuels used in dual fuel engines are predominantly hydrocarbons, some of these hydrocarbons will pass through the combustion chamber of the engine unreacted, thus retaining their original form in the exhaust stream. Other hydrocarbons will be partially combusted and also remain in the exhaust stream. Thus, the exhaust stream of a diesel or dual fuel engine contains measurable amounts of nitrogen oxides, carbon monoxide, and unburnt hydrocarbons.
It is imperative that when catalytic units are used with diesel or dual fuel engines to reduce exhaust emissions, the catalyst must remain effective over long periods of engine operation without significant degradation in catalyst activity. To accomplish this, a reducing agent, such as ammonia, introduced into the engine exhaust stream, must be carefully controlled in relation to the engine exhaust emissions. Otherwise, the reducing agent will be either ineffective or will contribute an added undesirable product to the engine exhaust stream emissions while having a possible detrimental effect on catalytic activity as well as good engine operation.
It has proven to be difficult to both economically and effectively control the use of a reducing agent with a catalytic reduction system for a diesel or dual fuel engine. There is a unique set of performance parameters specific to an engine which is determined by the engine build and which vary when the engine hardware is changed. To deal with these varying engine performance parameters which affect exhaust emissions, sampling devices are placed in the exhaust stream of an engine to draw samples to exhaust analyzers, such as a nitrogen oxide analyzer, to obtain measurements which can be used for control purposes. Systems of this type are both complex, expensive and require an operator who is a well trained technician.
It has been found that performance parameters of a specific engine can be mapped and stored in a central processor unit. This concept is disclosed by U.S. Pat. Nos. 4,368,705 to T. T. Stevenson et al., 4,619,234 to K. Okamoto, and 4,737,914 to K. Abe et al. The adaptation of an engine performance mapping technique for automated catalytic reduction would result in a system that is easy to maintain and operate.